1. Field of the Invention
This invention relates generally to the field of frequency synthesizer systems and more particularly to such systems for use in communication transceivers.
2. Description of the Prior Art
It is recognized that for many years there has been an ever increasing demand for FM two-way portable radios. This stems from the increasing use of portable radio communications in business, industry and government. The level of sophistication of the communication networks utilized by various institutions has constantly risen and many networks utilize some frequencies for local communications and other frequencies for longer range communications to a central location.
With the increasing use by many institutions of more sophisticated communications systems has come the demand for the capability of the portable transceivers to operate over a much larger number of frequencies. The majority of two-way FM portable radios currently in use today are those which employ conventional crystal controlled frequency channel elements. Thus, as each portable radio is required to either transmit or receive on more than one frequency an additional crystal controlled channel element must be included and electrically actuated so that both transmission and/or reception can occur.
As is the case with the several networks or systems, many different frequency combinations are grouped into communication channels from the available frequencies within a given area. Thus, any portable transceiver customer must specify to the manufacturer the desired pairs of receive and transmit frequencies for the several communication channels which are required in portable transceivers for a selected area. With such an order, the corresponding crystals to enable two-way communication on those channels must be manufactured and inserted into the transceiver. Frequently, design modifications must be made to accommodate the additional channel elements resulting in an increased size and weight of unit and a greatly increased cost.
The required long lead time for the manufacture of multifrequency transceivers to meet the market demand has caused a general trend toward the use of frequency synthesis in the design of FM two-way portable radios. A high order of accuracy of frequency control for a transceiver may be achieved by crystal control of the conversion oscillator. However, the multiple-frequency operation of the transceiver would call for a large number of crystals which is especially true where the transmit and receive frequencies are not identical. This difficulty may be reduced for certain frequency combinations by the use of a switched crystal frequency generator, a device in which the harmonics and subharmonics of one or more oscillators are mixed to provide a multiplicity of output frequencies. All of the resulting output frequencies are harmonically related to a subharmonic of the one or more master oscillators. The combination of a master oscillator signal with a secondary signal in a suitable mixer can provide the choice of a number of controlled frequencies. This reduces the number of crystals necessary to achieve several controlled frequencies.
However, there remains the difficulty of having only a restricted set of possible frequency combinations, whereas the customer may require unique combinations for his communication network. If a stable variable-frequency oscillator is substituted for a fixed crystal oscillator and a digital frequency synthesis technique is employed, a virtually unlimited number of discrete frequencies directly related to the frequency of the master oscillator are available. Instead of providing a plurality of individual channel elements suitable for each individual user's purpose, a manufacturer can provide one or more crystal controlled oscillators and a programmable memory which can be modified at the factory to conform to an individual user's required frequencies. This enables a manufacturer to assembly virtually all of his transceiver units in the same way and near the last step in the manufacturing process insert the memory programmed to the individual user's frequency requirements.
As will be described in greater detail, the programmable memory provides a series of numerical divisors which modify an output signal from a voltage controlled oscillator to cause tuning to any of a number of various frequencies. The use of digital frequency synthesizers is known in the art of radio transmitters and receivers but it has only recently been incorporated into the operation for two-way portable radios.
One of the basic problems with the use of frequency synthesized portable radios is the limited power available for the portable hand-held units. Thus, any frequency synthesis system must not be wasteful of the limited battery capacity available in the portable units. In addition, it has been found that various design implementations of digital frequency synthesizers do not meet the rigid specifications which are applied to radios which employ crystal controlled channel elements. It is well recognized in the art that crystal controlled channel elements produce extremely well defined frequencies. Digital frequency synthesizers can provide the same degree of accurate tuning as can the crystal controlled channel elements. But, the replacement of crystal controlled channel elements by a frequency synthesis system might ordinarily result in some degradation in the performance specification as, for example, adjacent channel selectivity.
The problem is therefore to find a digital frequency synthesized transceiver system for portable transceivers which will provide the size reduction, cost efficiency, power conservation and programmability that a multitude of users require yet can stay within the rigid performance specifications that apply to the use of crystal controlled elements.
Although too general and therefore not suitable to solve the problem, a block diagram of a known digital frequency synthesizer is shown in FIG. 1. A basic element of frequency synthesis systems is the phase-lock loop circuit in which the output of a voltage-controlled oscillator (VCO) is constantly compared with the frequency of the master crystal oscillator. Any unwanted change or drift in frequency of the variable controlled oscillator with respect to the master oscillator is detected by the phase comparator. When such a phase difference exists, the phase detector generates a control voltage which returns the VCO to the correct frequency.
Normally, the output signal of the master crystal oscillator is applied to a freqency divider that divides that signal by a fixed integer M and provides a square-wave output reference signal at 1/Mth the frequency of the master oscillator. Similarly, the output signal of the voltage-controlled oscillator is divided by a variable divider in which produces a signal at 1/Nth the frequency of the VCO. This signal is compared with a reference signal which may be a square wave from the fixed M divider in a phase comparator. Any phase difference is detected and applied through an integrating circuit and a low-pass filter to the voltage-controlled oscillator. This phase difference signal, after being processed and filtered provides a DC control voltage that is highest when the phase difference is greatest. When the signals are equal and in phase in the comparator, the loop is said to be "locked."
The output frequency of the synthesizer can be changed by varying the divide ratio of the variable divider N. When a new frequency is within the capture range of the phase-locked loop, the control voltage will change to bring the frequency of the VCO to the new value demanded by the setting of the variable divider. If a new frequency is outside the capture range of the circuit, the VCO will be swept through its entire operating range, and as the VCO frequency then enters the capture range of the phase-locked loop, the loop will take over frequency control and lock on the desired frequency.